1. Field of the Invention
This invention relates to a silicon carbide sintered body which is suitable for electrical discharge machining and which exhibits excellent high-temperature strength. It also relates to a method for manufacturing such a silicon carbide sintered body.
2. Description of the Prior Art
Silicon carbide sintered bodies have excellent oxidation resistance, corrosion resistance, heat resistance, thermal shock resistance, high-temperature strength, and other superior properties which make them highly suitable for use in parts for high-temperature gas turbines, parts for automotive engines, items requiring corrosion resistance, and the like.
Silicon carbide (SiC) has a relatively strong covalent bond, and therefore it is practically impossible to sinter silicon carbide powder alone. In order to obtain a strong, dense silicon carbide sintered body, use of various sintering or densification additives has been proposed in the prior art.
The most typical methods for manufacturing silicon carbide sintered bodies are the high-temperature pressurized sintering method (also known as the hot-pressing method) and the pressureless sintering method. In the latter method, one or more sintering additives such as a boron compound, an aluminum compound, and carbon or a carbonaceous substance are added to silicon carbide powder and the resulting mixture is shaped or molded into a green body and sintered in a non-oxidizing atmosphere at subatmospheric or atmospheric pressure.
Recently, much research has been performed with respect to sintering of SiC-Al, SiC-Al-C, SiC-B-C, and SiC-B-Al-C systems. Examples of patent publications which disclose sintering of these systems are U.S. Pat. No. 4,455,385 (disclosing pressureless sintering of an SiC-B-C system), U.S. Pat. No. 4,135,937 (hot-pressing of an SiC-AlB.sub.2 -C system), U.S. Pat. No. 4,354,991 and Japanese Patent Publication No. 60-34515 (both disclosing pressureless sintering of an SiC-Al.sub.2 O.sub.3 system), and Japanese Patent Laid-Open Applications Nos. 55-3396 (pressureless sintering of an SiC-AlN-C system) and 56-155071 (pressureless sintering of an SiC-Al-B-C system). As a result of the technical advances in this field, it has become possible to obtain high-strength and high-density silicon carbide sintered bodies by the hot-pressing method and the pressureless sintering method. However, particularly with respect to SiC-Al-C, SiC-B-C, and SiC-B-Al-C systems, there is the problem that abnormal or excessive grain growth tends to occur during sintering, and this may cause a deterioration in strength.
Furthermore, all the sintered bodies of the above prior art systems have the common problem that the electrical conductivity thereof is low, making it impossible to process them by electrical discharge machining.
Dense sintered silicon carbide products are most frequently used as substitutes for metallic parts, and they are usually required to have high dimensional accuracy. Since sintered SiC is usually prepared by powder metallurgy technology, i.e., by molding of powders followed by sintering, it is difficult to manufacture an as-sintered body having a complicated shape. Therefore, in most cases, silicon carbide sintered bodies are subsequently subjected to machining in order to obtain products having the desired shape and dimensional accuracy. However, as is well known to those skilled in the art, sintered SiC is a highly hard and brittle material, and it is extremely difficult to apply precise and fine work to it by means of a conventional mechanical machining technique such as grinding or boring. For this reason, precision parts, such as injection nozzles in which fine holes must be formed could not be manufactured from a sintered SiC body. In contrast with conventional mechanical machining, electrical machining techniques such as electrical discharge machining and electrochemical machining are suitable for precise and fine work of a very hard material such as sintered SiC. Therefore, it is highly desirable that sintered SiC be electrically conductive to such a degree that electrical discharge machining can be applied thereto.
Japanese Patent Laid-Open Application No. 57-22173 discloses a silicon carbide ceramic of an SiC-Al.sub.2 O.sub.3 -TiO.sub.2 system (containing 0.5-30% Al.sub.2 O.sub.3 and 0.2-15% TiO.sub.2) which can be subjected to electrical discharge machining. Japanese Patent Laid-Open Application No. 57-196770 discloses a sintered SiC product manufactured by electrical discharge machining in which one or more additives selected from carbides, nitrides, borides, oxides, or other compounds of Group IVa, Va and VIa elements (such as Ti, Ta, Hf, Zr, Mo, and W) and Al.sub.4 C.sub.3 are incorporated. U.S. Pat. No. 4,327,186 discloses electrically-conductive sintered articles based on a binary composition of SiC-TiB.sub.2, and more particularly made from an SiC-TiB.sub.2 -B-C system. However, these sintered products do not always exhibit satisfactory high-temperature strength, which is required for most uses of sintered SiC products.
Thus, there is still a need for a sintered body which has sufficient electrical conductivity to make it suitable for electrical discharge machining as well as improved high-temperature strength.